Electronically active guided-wave optoelectronic devices, such as lasers, photo-detectors and electroabsorption modulators fabricated from semiconductor materials, are able to conduct electrical current and transduce signals between the electrical and optical domains. These devices typically have high optical-propagation loss and couple poorly to optical fibers. Lenses are typically used to reduce the fiber-coupling loss. However, the need for lenses complicates the assembly/packaging procedure and greatly increases component costs. Electronically passive waveguides are preferably fabricated in dielectric materials, such as silica or lithium niobate, since they enjoy an order-of-magnitude lower propagation loss and because they couple efficiently with optical fibers. However, it generally is not possible to fabricate electronically active optoelectronic devices in dielectric materials, since they are insulators. Only optical filters with fixed response and modulators/switches based on temperature change or the electro-optic effect (which depends on an applied voltage rather than current) have been demonstrated in dielectric waveguides. The present invention can lead to the realization of electronically active guided-wave devices, such as those listed above, with dielectric waveguides that are low loss and have efficient coupling to optical fibers.
Many advanced optoelectronic devices and integrated photonic circuits, such as mode-locked lasers or optoelectronic transceivers, consist of various electronically active devices that are interconnected by passive waveguide networks. In general, the waveguides in these advanced devices are fabricated from a semiconductor material and thus tend to have a substantially higher loss and poorer coupling to optical fiber than do dielectric waveguides. In the prior art, in which the active devices are interconnected by dielectric waveguides, separate substrate chips of semiconductor or waveguide material must be optically aligned and properly butted against each other or coupled through optical lenses in order to function. The packaging effort associated with this method of combination is costly. The present invention achieves the integration of semiconductor active devices and dielectric waveguide networks on a single substrate. Also, multiple devices, each of which could contain multiple active elements, can be fabricated on a given substrate.
An example of a prior-art electronically active device that monolithically integrates a semiconductor waveguide with a photodetector is described in IEEE Photonics Technology Letters, v.5, pp. 514-517 (1993). The photodetector of this device is located above a portion of the semiconductor waveguide. The interface region between the photodetector and the waveguide layers is controlled because that interface region is produced as part of the single epitaxial growth for both the photodetector and waveguide materials. This approach has the disadvantage previously discussed above because it uses a semiconductor waveguide.
Examples of prior-art approaches for butt coupling of dielectric waveguides with lasers or photodetectors that are comprised of separate, complete chips are described in IEEE J. Selected Topics in Quantum Electronics, v. 6, pp. 4-13 (2000). A large variety of components have been realized with this approach. The difficulties of this approach are discussed above.
A prior-art approach that could be used to fabricate an electronically active waveguide device involves directly bonding two pieces of semiconductor materials. One piece contains fabricated semiconductor waveguides and the other piece could contain the active element. This approach is described in IEEE Photonics Technology Letters, v. 11, pp. 1003-1005 (1999). In this example, two semiconductor waveguides are bonded to a semiconductor microdisk resonator. The two semiconductor pieces are directly bonded together at high temperature (750° C. for GaAs materials and 400° C. for InP based materials). This approach also has the disadvantage previously discussed above because it uses a semiconductor waveguide.
A prior-art approach that could be used to bond thin electronically active device elements onto dielectric substrates is described in IEEE Photonics Technology Letters, v. 11, pp. 1244-1246 (1999). According to this approach, partially processed active device elements are bonded onto a carrier or transfer substrate by using a layer of organic polymer. Although GaAs was used as the transfer substrate in this prior-art example, other substrates such as quartz also could be used. In this example, the active device element is a guided-wave modulator that is based on semiconductor waveguides. The active device element is flipped top-side down above the transfer substrate. The top-side metal electrodes are fabricated on the active-device piece before it is bonded onto the transfer substrate and thus are located between the active-device epilayers and the transfer substrate. The semiconductor growth substrate for the active device element is then etched away and back-side metal electrodes are fabricated. In contrast to this prior-art approach, the present invention forms both the top-side and back-side metal electrodes after the active device pieces are bonded onto the dielectric waveguide material. This is because the interface between the active device piece and the dielectric-waveguide substrate of the present invention is used to couple light between the active device element and the dielectric waveguide and thus should be controlled carefully. The prior art approach does not involve coupling of light between the active device element and the transfer substrate (the substrate to which the active epilayer is attached temporarily).
Some of the examples of new devices that can be realized by the approach of the present invention involve microresonators whose properties are adjusted electrically. The closest prior art makes use of modifying a polymer overlay that is deposited above the microresonator to irreversibly trim the resonant wavelength of the microresonator. This prior-art approach is described in IEEE Photonics Technology Letters, v. 11, pp. 688-690 (1999). An advantage of the present approach over the prior art is that the electrical adjustment is reversible.